There are tens of thousands of estimated Hamas militants in a densely populated urban environment, and it's not like they walk around wearing uniforms or carrying Hamas ID cards. Israel doesn't even know where all of the 200+ hostages are, let alone every member of Hamas. The rich and powerful people who are leading Hamas are probably far from any real danger, and even if you managed to take a few of them out, someone down the totem pole would be perfectly happy to replace them.